Plasma Etching
Reproducible and controllable etching of 1um - 2um device features is almost impossible using wet etching techniques. RF plasma etching methods offer viable solutions to virtually all the problems that occur with wet etching. Mask adhesion appears to be less critical and undercutting can be controlled by varying the plasma chemistry. Materials used for dry etch methods can be quite corrosive and toxic but smaller volumes are required compared to wet etchants that can involve large volumes of hazardous acids and solvents. Moreover plasma techniques are well suited to the fully automated processing strategy required for large-scale device production.
Plasma formation
Applying a large electric or magnetic field to a volume
of gas generally creates the partially ionised gas made up of electrons,
ions and neutral species that characterises plasma. Different types
of plasma exist, but they vary firstly in electron concentration (ne),
and in average electron energy (kTe). High-pressure arcs typically
have ne = 
and kTe = 0.5 eV.
E / P
The ratio electric field strength E to pressure P is useful
in characterising the average electron energy in plasma.
As the electric field increases, free electrons, whose velocities
increase with the field strength, gain energy. Conversely, since the electrons can lose
energy by collisions, an increase in pressure, which decreases the mean free path of
the electrons, decreases their average energy.
Glow Discharges
The plasmas used for thin film processes in microelectronic applications
are characterised by pressures in the range of 50mTorr to 5Torr, electron densities
between 
and 
, and average electron
densities between 1 and 10eV (such energies correspond to temperatures of
- 
ēK). Glow discharges are
non-equilibrium plasmas since the electron temperature is greater than the ion or gas
temperature (Ti), typically Te / Ti > 10; they are also only weakly ionised since
the ratio of electrons to neutral species ne / nN is typically in the range
to 
.
Energetic electrons are capable of breaking chemical bonds via collisions with gas
molecules, generating highly reactive species for etching or deposition reactions.
Electrical Aspects
The energy of ions and electrons striking surfaces in a glow discharge is determined
by the potentials within the reaction chamber. The potentials of importance in rf glow
discharge systems are the plasma potential (the potential of the glow region),
the floating potential (the potential assumed by a surface within the plasma which is not
externally biased or grounded and thus draws no net current) and the potential of the
powered or externally biased electrode. When the plasma contacts a surface, that surface
even if grounded, is always at a negative potential with respect to the plasma.
Therefore positive ion bombardment occurs.
Plasma Etching Systems
A plasma etching system has several basic components: a vacuum chamber and a pumping
system to maintain reduced pressures; a power supply to create the glow discharge;
and gas or vapour handling capabilities to meter and control the reactant gases.
The detailed assembly of such a system depends on parameter space, parameter control
and application, but can be largely discussed in terms of three configurations:
barrel or volume-loaded; parallel plate or planar; and downstream.
The planar system can be operated in two modes.
