In
dry etching the substrates are placed into a vacuum chamber and exposed to
a plasma generated in a constant flow of specific gasses. The
material to be etched is then attacked chemically by reactive neutrals
(radicals) generated in the plasma.
Aggressive radicals can include fluorine or chlorine containing fragments
of the original gas molecules, but atomic oxygen or hydrogen as well.
If these radicals were to be the only etching elements, the etch would
proceed in an isotropic way.
Depending on the reactor design, however, the etch may be assisted by the
impact of energetic ions. Their unique direction of impact may give
an extra drive to the vertical etch rate, relative to the lateral
component. The technique is then called Reactive Ion Etching (RIE).
An important issue to consider is selectivity: the
etch rate of the mask material should be relatively low compared to the
material to be etched. The same applies to the etch rate of underlying
material in case the process has to stop at the interface.
The etch capability is limited to those materials that can be attacked in
an environment of chlorine-, fluorine-, oxygen-, or hydrogen/methane-
based radicals/ions and simultaneously produce volatile etch products.
If the material doesn't belong to this category, dry etching is still
possible in a physical way by virtue of the ion bombardment. The
process is then called sputter etching or ion milling.
A number of tools are available for purely chemical and isotropic etching,
mainly for resist stripping:
an Axcelis Fusion downstream etcher
and a number of barrel etchers, the Metroline/IPC being one of them.
Different geometries prevent the presence and influence of ions at the
location of the substrates.
There are three RIE-based tools available:
The main etch parameters are: gas mix composition, gas flow, process
pressure, RF power fed to the plasma, and some system-specific parameters
like RF bias voltage and process temperature.
The capability of each of these RIE system is intimately linked to its
specific design and every etch task is directed towards the system which
is most appropriate for that given task.
The range of materials that are regularly etched
comprises
Si, poly-Si, Si-oxide and associated materials
(e.g. SOG, TEOS), Si-nitride
metals (Al, Cr, Mo, Ti, Ta, W, etc.)
metal oxides and nitrides
polymers (e.g. resist, BCB, polymide)
Finally there is an Ion Beam Etch
(IBE) tool for those materials for which RIE is not a real option:
The range of materials that are regularly etched comprises:
In principle any metal or semiconductor
(e.g. Au, Pt, Ti)