7 Dry Etching for Micromachining Applications 415
(“matching unit”). The coil-and-capacitor arrangement is driven in resonance by
radio frequency (RF) power. In most cases, RF frequency of 13.56 MHz is used
for driving the ICP, but also 2 MHz and lower frequencies can be found in some
cases. In the ICP-configuration, the coil of one or several turns, depending on the
chosen frequency, is wound around a dielectric vessel. In the TCP arrangement, a
flat or dome-shaped coil covers a dielectric window, which may again be flat or
dome-shaped, on top of the etching chamber.
By feeding radio frequency power to the coil, a high-frequency oscillating mag-
netic field is generated inside the chamber, in the vicinity of the coil, with a
corresponding oscillating electric field resulting from Maxwell’s induction law.
The electric field can excite and drive a high-density plasma within the process
chamber. The plasma cannot shield itself effectively from this inner driving elec-
tric field by sheath formation, thus extremely high densities of charged particles
(electrons and ions ∼10
18
− 10
20
m
−3
) can be obtained by the inductive coupling
technology. Electrons are heated by the RF electric field and generate the ions and
chemically active species by collisions with gas molecules. However, before the
plasma of charged and neutral species reaches the substrate, the hot electrons need
to cool down and as a rule, the distributions of ions and neutrals requires some
kind of readjustment and rearrangement, for the homogenization of etching results.
Mechanical apertures [32], gas flow guides and gas flow redirectors, in combination
with balanced excitation of the inductive coil [33] and magnetic lenses for plasma
collimation [34], are found in this place.
Electron Cyclotron Resonance: This equipment is based on the fact that electro-
magnetic power can be propagated into the plasma more efficiently in the presence
of a magnetic field. Typically a cyclotron resonance condition can be established
when the frequency at which the electrons orbit around the magnetic field lines is
equal to the frequency of the applied electric field. In most cases, microwave fre-
quencies of 2.45 GHz or higher are used in ECR-equipment. Note that there is some
confusion with the use of the term “ECR” also for magnetically enhanced RIE con-
figurations (so-called MERIE or HRE), where plasma densities in a diode or triode
reactor configuration are enhanced to some degree by the interaction with stationary
or slowly varying magnetic fields.
In the ECR microwave system, the pressure should be low enough so that the
electrons can undergo several orbits without colliding with gas molecules (usu-
ally less than 0.132 Pa). One of the concerns with use of ECR-sources for RIE
applications is the nonvertical incidence of electrons and ions on the sample which
causes some isotropic behavior in the etch profile unless additional RF biasing is
applied to the substrate electrode for a more directional acceleration of the ions
to the wafer. The major drawback of ECR sources is the low operating pressure
window (0.0132–0.132 Pa) which limits the density of chemically active species in
the plasma and in many cases the achievable etching rates f or lack of reactants. At
higher pressure, electron cyclotron resonance disappears and microwave excitation
changes to an inefficient thermal heating of the plasma. On the other hand, in the
ECR mode, the degree of ionization and ion flux to the wafer are higher than with
other high-density plasma sources discussed, which makes the ECR an ideal source
for strongly ion driven reactions.