Download Characterization of low pressure plasma

Indian Journal of Pure & Applied Physics
Vol. 48, October 2010, pp. 723-730
Characterization of low pressure plasma-dc glow discharges
(Ar, SF6 and SF6/He) for Si etching
Bahaa T Chiada, Thair L Al-zubaydib, Mohammad K Khalaf a & Ausama I Khudiarc
a
Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq
b
Department of Materials Science, Ministry of Science and Technology, Baghdad, Iraq
c
Department of Laser and Optoelectronics, Ministry of Science and Technology, Baghdad, Iraq
E-mail: [email protected]
Received 26 June 2009; revised 14 July 2010; accepted 12 August 2010
Low-pressure plasma reactor which is generated for SF6, SF6/He and Ar gases discharges between two metal
electrodes (planer –parallel) using dc-high voltage power supply of 2 kV has been proposed. Paschen’s curves show the
breakdown voltage of gases as a function of the parameter p*d which is the product of the pressure in the chamber
(P=6.5×10−2-1.5×10−1 mbar) and the distance between the two electrodes (d=4.6 cm). The minimum breakdown voltages
were found 450 V at pressure of 1.35×10−1mbar and 276 V at pressure of 4.3×10−1 mbar for SF6 and Ar, respectively.
Current-voltage characteristics have been studied at different values of pressure (6.5×10−2-1.5×10−1 mbar) and interelectrodes spacing (3.4, 4.2, 4.6, 5 cm). The SF6, SF6/He and Ar gases discharges plasmas in Si etching have been discussed.
Keywords: Plasma etching, Si etching, Dry etching, SF6 discharge, Glow discharge
1 Introduction
Space and laboratory plasmas1,2 are classified by
their electron temperature Te, and charge particle
density n. A glow discharge is a kind of plasma
consisting of equal concentration of positive and
negative charges and a large number of neutral
species. The dc-glow discharge plasmas have been
used for plasma applications in the low and
intermediate pressure regions in modern technology.
Cold plasma technologies have been widely used in
industry for thin films deposition and surface
processing such as sputter- and plasma-assisted
chemical vapour deposition. Glow discharge plasma
is a tool for heating, sputtering, etching, nitriding and
ionization as well as an activator for gaseous atoms
and molecules. It is well known that a large number of
ions, electrons and excited radicals coexist in plasma.
These particles in plasma also have their kinetic
energy up to several hundred eVs, which may give
rise to physical and chemical effects in thin-film
formation and surface modification. A wide variety of
particles exist in the discharge in addition to ions and
electrons, including for example, radicals, excited
species, and various fractured gas molecules created
by collisions between electronics and gas molecules
or atoms. Overall, the discharge system must remain
electrically neutral even though some portions of it
are not3,5. One types of processing plasmas that is
relevant to this project is low-pressure dc-glow
discharges. The simplest configuration employed for
striking a gas discharge is a parallel electrode
geometry schematically shown in Fig. 1. Two plane
metal plates are separated by a distance, d, in a
chamber reactor filled with a particular gas at a
pressure, p. Breakdown of the gas is achieved by
applying an electric field of direct-current (dc) with
cathode and anode biased negatively and positively,
respectively. When the voltage between the plates is
low, the gas is a near-perfect insulator. As the voltage
is increased, a small fraction of electrons present in a
gas are accelerated towards the anode making
collisions with the background atoms. Some of these
collisions create positive ions which are then
accelerated towards the cathode. When the ions strike
the cathode, electrons are liberated from the metal
surface as a result of neutralization (secondary
electron emission). This process gives rise to an entire
avalanche of electrons leading to gas breakdown and
discharge formation. The voltage at which breakdown
occurs is described by Paschen’s law1,6. The
breakdown voltage is found to depend only on the
product pd for a given gas and cathode material. Plots
of the breakdown voltage versus pd are known as
Paschen curves. A characteristic minimum in the
function appears at some intermediate value of pd. At
low pd values, the breakdown voltage is high because
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INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 2010
Fig. 1 — Plane parallel electrode reactor for producing a dc glow
discharge
of too few collisions (low pressure or small gap). At
high pd values, the breakdown voltage is high because
of too many collisions (high pressure or large gap). It
should be noted that Paschen curves for different
gases or cathode materials will have roughly the same
shape but will be shifted from one another. Over the
past decades, plasma etching has been widely used in
the fabrication of silicon based integrated circuits.
However, due to complex physical and chemical
effects during etching, issues of reproducibility and
control of the interaction processes ultimately limit its
widespread application and further progress. In the
micro-electronics industry, glow discharge plasmas
are often used for etching of surfaces, in order to
generate topographical patterns on chips as an
alternative to wet chemical etching. Low-temperature
plasma processing offers important advantages over
wet-chemical methods. First, plasma-processing is dry
and safe. Secondly plasma-etching provides finer
resolution, sharper etching and less under cutting than
can be obtained with chemical ethantes. Thirdly,
plasma-processing makes possible to perform
sequential etching and stripping operations in the
same machine. Finally, plasma-processing creates no
pollution problems1,7. The anisotropy of wet etching
of mono characteristic crystalline silicon depends on
the crystal orientation. This means that the structuring
of the substrate is strongly related to this material
feature. When a specific profile is desired wet etching
is not appropriate and another process is needed.
Plasma etching as shown in Fig. 2, also called dry
etching, is a method for structuring the substrate in
the gas phase, physically by ion bombardment,
chemically by chemical reaction, or by combination
of both. Depending on the etching mechanism,
isotropic, directional, or vertical etch profiles can be
obtained8,9. Utilizing dry etching, the desired profiles
can be generated in polycrystalline as well as in single
crystalline and amorphous materials10. Different
etchant gases such as F2, CF4, SF6, NF3 and CLF3 as
fluoride atoms sources are used in plasma etching.
The SF6 has dielectric strength of about two three
Fig. 2 — Example of anisotropic (dry-etching) and isotropic (wet
etching)
Fig. 3 — Chamber of dc-glow discharges plasma
times that of air. It is non-toxic, non-flammable and
doesn’t react with other materials because it is inert
gas. SF6 and CF4 etchants are used in almost different
etching processes11-13. The properties of Si depend
strongly on the number of factor such as discharge
current, gas used and pressure. It is inevitable that
each etching technique with its associated controlling
parameters should yield etch samples with different
characteristics.
2 Experimental Details
A partial ionized plasma source of dc-glow
discharge at low pressures has been constructed as
home built plasma system, characterized and operated
at abnormal glow discharge. The plasma chamber
itself is a cylindrical stainless steel vacuum chamber
with length and diameter of 50 cm (Fig. 3). Within it,
there are two circular electrodes. Both the movable
electrode (denoted “Anode”) and the fixed electrode
CHIAD et al.: CHARACTERIZATION OF LOW PRESSURE PLASMA
(denoted “Cathode”) have a diameter of 14.5 cm.
Both are made of stainless steel and have exterior
high-voltage connections. A front view image of the
electrodes is shown in Fig. 4. A system body
grounded shield, also made of copper, connected to
anode electrode. The feedstock gas is supplied
through a gas regulator and kept at a preset pressure
by a membrane valve. The experiments are performed
using two-stage vacuum pump (mechanical and turbo)
to evacuate the plasma chamber down to 10-4 torr.
Argon, SF6 and SF6/He are used as plasma gases
during measurement to avoid possible negative effect
of complex chemical reaction when non-inert gas is
applied. Figure 5 shows a photograph of the main
experimental set-up used in this work.
The I-V, I-P measurements and Paschen's graph
refer to dc characterizations of device, the purposes of
Fig. 4 — Anode and cathode electrode-dark space holder
assembly
Fig. 5 — Low pressure (dc-glow discharge) plasma system
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performance analysis and parameter extraction. A
typical electrical circuit set-up for forming planar
discharges is schematically shown in Fig. 6. The dc
characterization set-up essentially consists of power
supply (4 kV), digital multimeter (discharge current
meter), digital multimeter (cathode voltage meter),
pirani-guge and reader. To obtain I-V curves, the
discharge was first ignited by providing over potential
with the dc power supply. The power supply voltage
was then adjusted to vary the discharge current. The
main characteristics of plasma discharge such as the
breakdown voltage pd, I-V and I-P characteristics
depend on the cathode voltage used, the gas pressure
and inter-electrode spacing in the chamber.
Plasma etch equipment can be divided into two
classifications. The first is the parallel-plate diode
configured etches system, where the wafers are
positioned on the electrode surface. The second
classification is the high-pressure plasma etcher
where the wafers are positioned in a rack that enables
the wafers to float in the plasma. The present work
represented the diode-configured system concept2-3.
The etching experiments were performed in a
conventional reactive ion etching RIE system with a
modified cathode .The cathode set-up used for the dc
discharge experiments is shown in Figs 4 and 7. The
cathode electrode (14.5 cm diam) was surrounded by
a dark space shield also serving as a gas. In order to
avoid arcing; a ceramic plate (3mm thick) was
inserted between the cathode and the grounded dark
space shield. The discharge was truncated by desired
masks. This arrangement defines the etching area and
masks it possible to calculate the current density. The
Si specimens were masked with 1 mm thick desired
pattern of lines and spaces and pinhole as shown in
Fig. 7.
The Si wafer for plasma etching has been cut into
pieces (10×10 mm) with 0.5 mm thickness using
diamond tip cutter. Silicon substrates are cleaned
thoroughly before putting into vacuum chamber. The
Fig. 6 — Electrical circuit used to generate and analyze the glow
discharge
INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 2010
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Fig. 8 — Plot of the breakdown voltage of Ar and SF6 versus
pressure and electrode spacing
Fig. 7 — Top and side view of the cathode set up
system is fitted with Ar, SF6 and SF6/He gas sources
and is capable of delivering up to 1.3 kV dc voltage.
The specimen (wafer) to be etched is placed on
cathode surface in the center of the chamber. It is then
evacuated to pressures better than 1×10−4 mbar using
a mechanical rotary pump and a turbo pump; the
etchant gas is then flowed in at a constant rate
regulated by flow controllers. The dc power supply is
then switched on to start the etching for constant
process time (1 h). Argon as discharging gas was
injected for cleaning the samples surface for 30 min
process time with cathode voltage of 1.3-1.5 kV. The
etch rate was calculated as the ratio of the etched
depth to the total etching time. Moreover the etched
profile was examined with optical microscope. The
etch depth of the treated Si samples was determined
by using the gravimetric method. Four digital balance
was used to found the weight difference between the
Si sample which plasma etched and untreated. We can
calculate the resulting etch depth by using the
following formula:
D=∆m/Aρ
where the ∆m is the sputtered material weight, A is the
active etched area and ρ is the material (Si) density.
3 Results and Discussion
3.1 Paschen’s law graphs
The results of our Paschen’s law experiments are
shown in Fig. 8 for planar parallel electrodes, at fixed
inter-electrode spacing (4.6 cm). The graph shows the
minimum voltage that can be expected to produce a
sustained glow discharge as a function of the gas
pressure (6.5×10−2-5×10−1 mbar) multiplied by the
distance between electrodes (4.6 cm). Figure 8 shows
the minimum breakdown voltages as 455 V at
pressure of 1.35×10−1 mbar and 276 V at pressure of
4.3×10−1 mbar for SF6 and Ar, respectively. The
breakdown voltage can be increased by reducting the
glow parameter pd, where further reduction of
pressure is not practicable, the same result can be
obtained by reducing the effective inter-electrode
spacing. The breakdown applied voltage up to
Paschen minimum value was found to increase with
increasing the pd parameter. These results are related
to the dependence of probability on the number of gas
molecules between electrodes at fixed temperature,
which is formally developed by Paschen’s law1.
3.2 I-V and I-P characteristics
The
characteristics
of
glow
discharge
current/voltage and pressure have been established
with the scheme shown in Fig. 9. The current in the
external circuit can be measured as a function of the
voltage drop between the anode and the cathode. A
further decrease in the external current limiting
resistance brings the voltage/discharge current
characteristic into the abnormal glow discharge
region. Since the visible glow already covers the
entire work surface, an increase in current density will
now be accompanied by an increase in the voltage
drop through the resistance of the glow discharge.
CHIAD et al.: CHARACTERIZATION OF LOW PRESSURE PLASMA
Fig. 9 — I-P characteristic of SF6 discharges plasmas
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These positive characteristics and behaviour attributed
to the mobility limited version of the Child-Langmuir
equation, where the current density is proportional to
V2 and inter-electrode spacing2-11 (d). The currentpressure (I-P) characteristics can give valuable
information about its mechanism. Figure 10 shows
discharge currents plotted against pressure using
different cathode applied voltages. Breakdown
voltage is found to decrease from approximately 1000
volt at (3.7 ×10−2 mbar) to 650 volt at (5×10−2 mbar).
The increment of gas pressure leads to increase of the
electron collisions (reduction of free path of electron)
with gas molecules, which means increasing the
ionization rate. The effective plasma resistance, Reff
can be estimated at the various pressures by finding
the slope of the I-V plots. From this calculation, Reff
is found to decrease from (30) KΩ at 6.5×10−2 mbar to
Fig. 10 — I-V characteristics of SF6 discharge plasmas at different value of gas pressure and inter-electrode spacing
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(8) KΩ at 1.5×10−1 mbar. Evidently, as the pressure is
raised the discharge glow column becomes more
conductive. The ion and electrons currents are usually
interpreted in terms of gas pressure; it is more
accurate to consider the gas density.
3.3 Etch results
The results discussed below are limited to Si which
is widely used in the semiconductors industry today.
Gases of SF6, SF6/He and Ar have been tried in
reactive plasma process. Typical dc glow discharge
I-V curves for the SF6, SF6/He and Ar etching gases
are shown in Fig. 11. The plasma etching photographs
Fig. 11 — I-V characteristics of Ar, SF6 and different percentage
(SF6/He) discharges plasmas
of the n-type Si surface are shown in Figs 12-15. The
gases pressure and process times were held constant
for these experiments. From resulting data of I-V
characteristic of plasma discharges for SF6, SF6 /He,
and Ar gases, we found that the current density value
of SF6 discharges is maximum in comparison to
others for limited area of Si wafer. This result implies
that at given pressure and bias potential, the etching
rate increases linearly with increase of ion current.
From Fig. 12 of dc Ar glow discharge (n-type Si
etching photographs) and the weight difference
measurements of samples as untreated and plasma
treated, we concluded that there is no distinguished
etching. This result of dc Ar glow discharge can be
used for surface cleaning as primary step to etch
processing of other gases. SF6 plasma were observed
to etch Si with high etching depth which attributed to
long inherent lifetime for F atoms and their high
reactivity with Si (Ref. 12) as expected (Fig. 13). The
Fig. 12 — Plasma etching of Ar-discharges
CHIAD et al.: CHARACTERIZATION OF LOW PRESSURE PLASMA
Fig. 13 — Plasma etching of SF6 discharges
results indicated that the etch depth is linearly
proportional to the glow discharge current density and
the SF6 gas pressure. The process of SF6 discharges
gives an etching profile with good agreement images
of masks. The resultants provide sharper etching and
more resolution than obtained with chemical
etchants14. The He gas was mixed with SF6 in order
to sustain a uniform and stable plasma. The n-type Si
etching rate increases for low He concentration, as
shown in Fig. 14(a,b) for SF6(90)/He(10) and
SF6(80)/He(20), while etch rate becomes very low at
high He concentration of 40%. The observations of
Fig. 14 indicated that the etched depth has been
improved for the calculated etch rate of 0.1-0.2
µm/min. The etch rate depends on the free fluoride at
low concentrations of He, as SF6 pressure increases
the etch depth will also increase. The same results are
obtained for Si etch rate as mixing of CF3Br and He
Fig. 14 — Plasma etching of SF6/He discharges (low He
percentage)
Fig. 15 — Plasma etching of SF6/He discharges (high He
percentage)
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INDIAN J PURE & APPL PHYS, VOL 48, OCTOBER 2010
and as a function of separated and combined beams of
XeF 2 and Ar (Refs 15,16). Etch rate is very low at
high He concentration of 40% for many reasons, such
as the reduction of fluoride atoms, the oxide and
polymerized layers production. Polymerization
usually resulted depending on some other plasma
parameters, deposition occurred rather than etching 15,
as shown in Fig. 15.
4 Conclusions
The visualization of I-V and I-P characteristics
shows that the electrical discharges plasmas are
operated in abnormal region which is effective
parameter of surface processing of Si wafer. Results
of Ar dc-glow discharges can be used for surface
cleaning as primary step to etch processing of other
gases. SF6 plasma were observed to etch Si with high
rate while mixing with low percentage of He gas leads
to uniform and stable plasma without increasing the
etch rate rather than SF6 pure.
References
1 Lieberman M A & Lichtenberg A J, Principles of Plasma
Discharges and Materials Processing (John Wiley, New
York), 1994, pp 8-10.
2 Chen F, Introduction to Plasma Physics and Controlled
Fusion, Plenum Press (New York), 1984, p 13.
3 Brown S C, Introduction to Electrical Discharges in Gases
(John Wiley, New York), 1966, p 301.
4 Manos Dennis, M. & Flamm Daniel, L. (eds), Plasma
Etching–An Introduction, (Academic Press, CA), 1989, pp
25-45.
5 Joseph C, in Handbook of Plasma Processing Technology
(eds Rossnagel Stephen, Cuomo Jerome, J. and Westwood
William, W.) (Noyes Publications, Park Ridge, N J), 1990,
pp 140, 196.
6 Ledernez L , Olcaytug F , Yasuda H & Urban G , ICPIG
29th, 2009, p 12.
7 Bersin R L, Solid State Technol, May (1976) 32.
8 Chapman B, Glow Discharge Processes: Sputtering and
Etching (John Wiley & Sons, New York), 1980, p 299
9 Manos D M & Flamm D L, Plasma Etching: An Introduction
(Academic Press, New York Manos), 1989, p 29
10 Bogaerts, Neyts E & Gijbels R, Spectrochimica Acta Part B,
57 (2002) p.609.
11 Flamm D L, Donnelly V M & Mucha J A, J Appl Phys, 52
(1981) 3633.
12 Vasile M J & Stevie F A, J Appl Phys, 53 (1982) 3799.
13 Yan J D, Fang M T C & Liu Q S, IEEE Dielectrics and
Electrical Insulation, 4 (1997) 1.
14 Bondur J A, J Vac SciTechnol, 13 (1976) 1023.
15 Mogaband C J & Evinstein H J L, J Vac Sci Technol, 17
(1980) 3.
16 Coburn J W & Winters H F, J Apply Phys, 50 (1979) 3189.