st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Experimental investigations on helium APPJ characteristics and
effects on photo-resist etching
Wenjun Ning, Lijun Wang, Mingzheng Fu, Shenli Jia
State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University,
Xi’an, 710049, China
Email: [email protected]
Abstract: He-APPJ generated by 10 kHz sinusoidal voltage source in a quartz tube was studied on
its electric and optical properties and then applied to Photo-resist (PR) etching. Despite the stable and
uniform appearance, the discharge was actually composed by a series of fast ionization waves with the
maximum speed of ~104 m/s. Optical emission spectrum (OES) contained rotational band of OH and
second positive system of N2, which could be used to estimate the gas temperature and electron temperature. PR could be removed by He-APPJ, which might be mainly contributed to simple physical
impaction rather than chemical collisions, because the energy of detected emission photos was insufficient to break the carbon-bonds such as C-H, C-C and C-O.
Keywords: He-APPJ, plasma bullet, Optical emission spectrum, Photo-resist etching.
1. Introduction
The process of stripping unnecessary photo-resists
was important in large scale integrated circuit (LSIC)
fabrication industry. Conventional ways such as low
pressure plasma or wet chemicals for photo-resist removal had some essential drawbacks: the low pressure
plasma called for expensive vacuum system while the
wet chemicals could cause environment issues [1]. As a
promising substitution, atmospheric pressure plasma jet
(APPJ) had proved to be clean, safe and economic, which
was widely investigated on variety applications in the
areas of medical treating and material processing in recent years [2].
Several devices had been developed for generating
APPJ [2]. APPJ was a kind of highly-collisional micro-discharge in which energetic species quenched by
molecules from ambient air, thus the non-equilibrium
state of plasma was sensitive to the injection power, the
electrode structure, the working gas and the flow rate [3].
Electrodes with large curvature such as needle or fine
wire driven by high frequency or short pulsed voltage
were apt to ignite plasma, but the induced high energy
ions usually caused ion bombardment damage to the silicon wafer [4]. Dielectric barrier discharge (DBD) could
prevented from intense discharge such as spark or arc,
meanwhile large amounts of active species could be carried by working gas downstream to several centimeters,
which made it suitable for sensitive material processing.
Noble gases were the preferred working gases for their
comparatively stable chemical forms, since monatomic
gas only had excitation level, more energy could be
transferred to electrons and ions.
In this study, we investigated He-APPJ on its electric and optical properties, and then it was applied to etch
photo-resist coated on silicon substrate. Polarizing microscope and X-ray photoelectron spectroscopy (XPS)
were used to study the etching effect.
2. Experiment setup
The applied APPJ system was a DBD structure,
which contained a quartz tube as the dielectric barrier
and gas channel with inner and outer diameters to be
4mm and 6mm, respectively. Two coaxial copper sheets
wrapping around the tube separated by 2mm were used
as the electrodes, which connected directly to a 10 kHz
sinusoid voltage source since matching network was unnecessary in this case. Highly pure helium (99.99%) was
controlled by mass flow meter with constant flow rate of
3000 standard cubic centimeter per minute (sccm). Optical spectrometer (Andor SR750, Andor Ltd, UK)
equipped with iCCD (Andor’s iStar, Andor Ltd, UK) was
used to detect the active species in plasma and capture
the temporal and spatial development of the discharge.
Current in the circuit was taken from a sample resistance
(50 Ω) in series with the ground electrode, while the input voltage was monitored by a capacitive divider in series with the power electrode. An oscilloscope (DPO
3014, Tektronix Inc., USA) was employed to evaluate the
I-V curve; moreover, with the help of digital delay generator (DG535, Stanford, USA), it gave the trigger signal
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
to iCCD based on the current waveform. The system was
illustrated in Fig. 1.
3. Results and Analysis
3.1 Electric property
Based on the Paschen’s law, helium can be ignited
in relatively low voltage (Vp-p=6 kV in this case). With
the increasing of power supply, the APPJ could go
through three distinct modes, namely the chaos mode,
bullet mode and the continuous mode [5]. In this studied
case bullet mode was preferred since chaos mode was
weak and unstable, whereas continuous mode could easily transfer to spark discharge because of plasma heating.
Fig. 1 showed the I-V curves of the jet operating in bullet
mode. DBD structure assigned the capacitive nature of
discharge that the phase shift was around 90o, even when
the discharge was ignited; Discharge current which was
defined as the subtracting displacement current (plasma
off) from the total current (plasma on) showed two isolated peaks in a voltage period, which indicated that the
discharge was discontinuous. Displacement current was
the main component in total current, thus the discharge
was stable and low power consumed, which was approximate 0.5W as shown in Fig. 2.
3.2 Optical investigation
Despite the uniform and stable appearance of the
He-APPJ, research had pointed out that the discharge
was actually a series of ionization waves travelling with
speed of 104~105m/s ,known as the ‘plasma bullet’[2].
With the help of intensified Charge Coupled Device
(iCCD), the temporal and spatial evolution of He-APPJ
could be clearly observed. Fig. 2 (a) showed the plasma
bullet evolution during the time section ∆t1 corresponding to Fig. 1. It was worth noting that the bullets was
firstly generated on both edge of the power electrode,
and then travelled independently into the DBD zone or
the open space downstream. Plasma bullet was actually
ionization wave which was familiar with positive
streamer; Large amounts of positive ion moved fast towards the cathode and collided with molecules and metastable species, while electrons quickly recombined with
positive ions behind the streamer head to form quasi-neutral plasma channel. The applied voltage then
mainly dropped at streamer head, resulting in very high
electric field. There were different opinions on the
mechanism of positive streamer propagation; Lu et al
proposed a photo-ionization model which emphasized
the importance of photo-ionization before the streamer
head supplying seed electrons [6], while Sakiyama et al
argued that the Penning ionization might play a crucial
role [7]. Optical emission spectrum (OES) shown in Fig.
3 supported Sakiyama’s theory. The first negative system
(FNS) of N2+ at 391.4nm was detected, which was generated by Penning process:
He*+N2=>N2++He+e
(1)
He2*+N2=>N2++2He+e
(2)
Fig. 2 (b) was the measured bullet speed in the jet
where the zero position corresponding to the exit nozzle.
The bullets were firstly accelerated by the electric field to
a maximum speed of 1.38x104 m/s, and then slowed
down until completed quenched by the ambient air. The
jet length in our study was about 1.5cm, which could be
manipulated by the input power, the gas flow rate and the
tube size [8].
Because of the interaction with ambient air, abundant of N-contained spectrum lines were detected in the
OES; In addition, OH (308.4nm) line was obvious, since
the moisture in air could be decomposed in collisional
process. The electron temperature and gas temperature
were then evaluated from the vibrational level of N2 second positive system (SPS) and the rotational level of
OH, respectively [9]. Spectrum Analyzer 1.7 was used to
calculate the spectrum assuming the populations following Boltzmann distribution [10].Three emission lines of
N2 (C→B, ∆ν=-2) at 371.05nm, 375.54nm, 380.49nm
were used for vibrational temperature, and four emission
lines of OH (A → X, ∆ν=0) at 307.84nm， 308nm ，
308.33nm and 308.52nm was used for rotational temperature. Under the experiment conditions mentioned above,
the electron temperature and gas temperature were estimated to be around 3000 K (0.26 eV) and 320 K by. The
results revealed that He-APPJ was typically
non-equilibrium plasma, which was suitable for sensitive
material interaction.
3.3 Photo-resist Etching
Commercial available EPG-533 negative photo-resist
coated on silicon substrate with thickness ≈ 1µm was
used in the experiment. When silicon substrate was
placed underneath the He-APPJ, the discharge was more
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
intense and the interaction area was concentrated, which
could be explained by that the silicon wafer acted as
suspended electrode thus the electric field was intensified
in the jet [11]. The distance between the exit nozzle and
the substrate was maintained to be 5 mm. Fig. 5 showed
the surface morphology of the photo-resist after 5
minutes etching. Photo-resist was removed from the
central spot out spread to about 0.85 mm, resulting in
ring-shape appearance. The surface was clean and little
ion bombardment burning was observed.
PR was mainly composed by C, O and H, the
mechanism of He-APPJ removing PR could be explained
as follows. The detected emission photos (Emax=4.1eV at
308nm) were insufficient to break the carbon bonds such
as C-O (3.6 eV) , C-C (3.8 eV) and C-H (4.2 eV) [12], so
it was reasonable to assume that simple physical impaction was the dominated process. As mentioned before,
He-APPJ was composed by a series of high-speed (104
m/s) ionization waves, the kinetic energy carried by ions
such as N2+ and He+ was sufficiently high (estimated to
be hundreds eV) to scissors the carbon bonds, then O
radicals combined with hydrocarbon fragments to produce volatile CO2 and H2O. After the etching, XPS analysis revealed that C was reduced, whereas N, O and Si
were increased. During the photo-resist etching process,
C was converted to volatile CO2 and silicon was exposed
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
to the air; On the other hand, N and O could be trapped
by complex N, O-containing structures such as C-NO
bonds [1].
References
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4. Conclusion
He-APPJ was investigated on electrical and optical
properties and the effect on PR etching. The discharge
was actually discontinuous but a series of fast ionization
waves (known as the ‘plasma bullet’) with the maximum
speed to be 1.38x104 m/s in our device. Power consumed
in the discharge was relatively low and the gas remained
approximate room temperature, which was suitable for
heat-sensitive material processing. He-APPJ was able to
remove PR with little burning damage left. The optical
emission spectrum could only detect lines with energy
that was insufficient for breaking carbon bonds such as
C-H and C-O, so it was reasonable to assume that the
dominant mechanism of PR etching by He-APPJ was
simple physical impaction.
Acknowledge
This research was supported by the Fundamental
Research Funds for the Central Universities of China.
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