ESTIMATING ELECTRONS AND IONS ENERGIES IN AN RF
CAPACITIVELY-COUPLED ARGON DISCHARGE*
M. AFLORI, L.M. IVAN, M. MIHAI-PLUGARU, D.G. DIMITRIU
Department of Plasma Physics, Faculty of Physics, “Al. I. Cuza” University of Iasi, 11 Carol I Blvd.,
RO-700506 Iasi, Romania, e-mail: [email protected]
Received December 21, 2004
From a fundamental standpoint, the dry etch process [1-3] can be divided
conceptually into three separate inter-linked areas: the glow region of the plasma
produced in the device, the sheath field which exist between the glow and the sample,
which is sited on an electrode surface and the interaction between species generated in
the plasma and the sample to be etched. A completely characterisation of a dry etch
process demands an investigation of each of these three areas. The first area, the glow
region acts as a source of reactive neutral and ionic species. The neutral species diffuse
to the electrodes and the ions are accelerated to the electrodes by the sheaths fields.
The purpose of this work is to investigate the glow region using Langmuir probe
with an RF-compensation electrode for argon radio-frequency plasma in a device that
consist of asymmetrical electrodes. The system used was an asymmetrical (the powered
electrode being much smaller than the earthed anode which included the chamber
walls) industrial OPT Plasmalab 100.
The ion energy at the surface of the quartz plate could be estimated by
measuring the potential difference between the plasma and the quartz plate situated on
the powered electrode [2, 3].
The electron energy distribution is automatically calculated using the soft supplied
with the probe, for a range of pressures 60-80 mTorr and powers 10–100 W. Those
distributions illustrate the presence of two groups of electrons, with different energies.
Key words: Langmuir probe, argon radio-frequency plasma, electron energy distribution
function.
INTRODUCTION
Plasma processing is an important branch of industrial micro-fabrication and
in particular reactive ion etching is generally used for resist etching [1]. An
understanding of the physical processes occurring in plasma is necessary, because
only on this basis can the technique be advanced and used in an even-widening
range of applications.
*
Paper presented at the 5th International Balkan Workshop on Applied Physics, 5–7 July
2004, Constanţa, Romania.
Rom. Journ. Phys., Vol. 51, Nos. 1–2, P. 225–230, Bucharest, 2006
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RF plasmas are more efficient in converting the power from the supply into
the plasma and more importantly in etching semiconductors than DC plasmas [2,
3]. They are also capable of producing much lower energy ions at the cathode than
in the DC case which reduces the damages caused to the sample. The device is of
asymmetric type with chamber wall constituting the grounded anode and the
cathode being driven. The cathode is capacitively coupled to the power supply in
order to allow negative DC bias to build up on the cathode. The hanging RF field
gives energy to the electrons, whereas the slower moving ions are given energy by
the DC bias at the cathode.
Argon is a chemically inert gas, common constituent of etching plasmas and
widely used in sputtering applications. Langmuir probe diagnostics methods are
useful for measuring plasma parameters in low-pressure gas discharges [4-11].
EXPERIMENTAL SET-UP AND RESULTS
The system used was an asymmetrical (the powered electrode being much
smaller than the earthed anode which included the chamber walls) industrial
Oxford Plasma Technology “ Plasmalab 100+” system with a Hiden Analitical RFcompensated Langmuir probe inserted into the middle of the plasma, as shown in
figure 1. The quartz plate shown in the diagram was 12 mm thick and covered the
cathode with the exception of a ring approximately 5 mm wide at the cathode’s
edge. Both cathode and anode are made from aluminium. The distance between
probe tip and quartz plate was 18.5 mm.
Fig. 1 – Diagram of the asymmetrical plasma device.
3
Electrons and ions energies in an argon discharge
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Fig. 2 – Plasma potential versus gas pressure for different input rf power.
Initially, the potential at the cathode is sinusoidal with an average value of 0
V and peak to peak value up to several hundred volts. As the potential at the
cathode begins to go positive, a cascade caused by secondary ionizations appears
and the highly mobile electrons collide with the cathode, lowering its potential and
therefore causing a potential across the capacitor between the cathode and RF
supply. The ions are accelerated away from the cathode, but the distance on they
move is negligible before the field reverses direction. On the next half cycle, the
cathode begins to be negative and electrons are accelerated away, increasing in this
way the value of the negative potential. The cathode becomes negative for most of
the cycle and ions are accelerated towards it for most of the time. Due, therefore, to
the higher mobility of the electrons, over subsequent cycles the cathode acquires a
DC bias consistent with the flux of electrons and ions to the cathode being equal in
subsequent half cycles. The bulk of the plasma is equipotential and therefore is
field free, again with most of the potential lost over a small region known as the
dark space or sheath above the cathode [2].
When equilibrium is reached, the rate of ionisation is equal to the rate of
recombination (mainly at the walls) and the power into the system is equal to the
power dissipated (by heating the chamber, electrode and gas and in
photoemissions). There are two methods by which the supply powers the plasma.
The first is by primary electrons which exist in the plasma and bounce between the
relatively negatively charged walls and cathode. The second is by secondary
electron emission.
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Fig. 3 – EEDF for argon, 60mTorr.
Fig. 3 demonstrates the evolution of the EEDF as the power is changed from
10 W to 100 W, for plasma potential shown in Fig. 2. In Fig. 3 the electron energy
distribution was automatically calculated for argon plasma. This figure illustrates
the presence of two groups of electrons, with different energies. The first group are
thermalized to follow an isotropic Maxwellian population, but not in complete
thermal equilibrium with the cooler background electrons. The second group
constitutes the bulk of plasma density and they arise from collisional ionization of
the neutral gas by primary discharge electrons [12]. The energy of both groups of
electrons is only slightly influenced by the rf powers values, for a given pressure.
In the absence of the quartz plate, the capacitively coupled cathode attains a
D.C. bias, which depends on the relative mobility of the ions and electrons and
their relative density. Quartz is used because it does not introduce contaminants to
the chamber as other surfaces would. The quartz platen is circular and of radius
slightly smaller than the cathode such that it covers almost all but leaves an annular
ring of several millimeters cathode exposed directly to the plasma. The
introduction of this slab of dielectric can be expected to have a non-eligible effect
on the potentials in the system due to this conductivity and secondary electron
emission function. If the quartz acted as a perfect insulator and shielded the
cathode completely, the large D.C. bias which is a result of charge built up at the
surface would not result in a D.C. bias at the backing plate (because it is not
possible to draw a net D.C. current trough the insulating target plate) [2].
Layberry [2, 3] had measured the maximum negative potential at the surface
of the quartz in the same device as a function of the power and pressure. The
maximum energy of ions incident at the cathode, assuming transport across the
cathode sheath to be collisionless, is [3]:
5
Electrons and ions energies in an argon discharge
Ei ,max ~ q (V p − Vq )
229
(1)
where Vq is the bias potential at the plate surface and q is the charge on the ion.
Fig. 4 – Difference between plasma potential and quartz plate potential.
Fig. 5 – Ion densities versus rf power and gas pressure.
The ion energy at the surface of the quartz plate could be estimated by
measuring the potential difference between the plasma and the quartz plate. Fig. 4
shows estimated difference between plasma potential (calculated for argon, Fig. 2)
and quartz plate measured by Layberry et al. [2, 3].
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Using Langmuir probe, densities of ions can be automatically calculated. The
results are presented in Fig. 5. The ion density measured in the plasma bulk is
increasing with both power and pressure. With increasing of pressure, the plasma
density increases and the frequency of collisions became higher. Electrons suffer
collisions with neutral particles, ionizing them. They lose their energy and are
accelerated in rf field gaining again energy to produce ionizations.
CONCLUSIONS
We have demonstrated the existence of two electron groups. Electrons in the
first group, with low energy, affect local electron densities and local plasma
conductance. Electrons in the second group, with high energy, play the main role in
the local excitation and the local ion production. They effectively interact with
argon atoms in elastic and ionization collisions and compensate their energy losses
trough stochastic heating on the oscillating plasma-sheath interfaces.
The ion energy at the surface of the quartz plate was estimated by measuring
the potential difference between the plasma and the quartz plate. The ion density
measured in the plasma bulk is increasing with both power and pressure.
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