TEGAL 9XX Plasma/RlE Etch Systems are used by the Semiconductor
Industry for integrated circuit fabrication. The systems are used
in one part of the sequence of manufacturing steps that transfer
a pattern formed from a layer of photosensitive material, the
photoresist, to a layer that makes up a permanent part of the
finished device. The process of defining a pattern with photoresist
is known as photolithography, while the etch process transfers
the photoresist pattern to the permanent layer.
The materials used in semiconductor
device fabrication may be etched in two ways, either wet or dry.
In wet etching, the material to be etched comes into contact with
a liquid, in which the material dissolves. The action of the liquid
solvent removes material that is exposed to the solution. Material
that is masked, or covered, by the photoresist remains after etching
as a permanent pattern. Dry etching, also termed plasma etching,
substitutes a reactive gas mixture for the liquid solvent to accomplish
the same result, that of pattern transference. Dry etching is
capable of transferring smaller features into the permanent layer
than wet etching, with greater control over the variation in feature
size. The current
requirements of the Semiconductor Industry necessitate the use
of dry etching for most of the pattern-transfer steps. As semiconductor
devices become denser and faster, the shift to dry etching will
Dry etching systems are divided into two broad categories: batch
etchers and single-wafer etchers. Batch etching systems etch more
than one wafer at a time, while single-wafer systems process just
one wafer to completion before proceeding to the next.
The TEGAL 9XX Plasma/RlE Systems
are dry, single-wafer etchers. Wafers in the 9XX are transported
to a Reaction Chamber. A gas mixture is introduced into the Reaction
Chamber, and the gas mixture is caused to become reactive by the
application of radio frequency (RF) electromagnetic radiation.
The reactive mixture, or plasma, etches away material that is
not covered by the masking photoresist. The etch process is terminated
at an appropriate time, the wafer is unloaded from the Reaction
Chamber, and a new wafer is introduced. The cycle repeats. The
general mechanisms by which etching proceeds in a plasma etching
system are as follows: RF power accelerates free electrons in
a low-pressure gas mixture. The accelerated electrons undergo
collisions with gas molecules and these collisions result in the
generation of several new species. If the gas molecules are broken
apart, or dissociated, free radicals are formed. Free radicals
chemically reactive molecule fragments with no net electrical
charge. Radicals that come into contact with material on the wafer
surface may be sufficiently reactive chemically to combine with
the surface to form volatile reaction products. The gas molecules
may be dissociated and ionized. If they are, the molecule fragments
have a net electrical charge and respond to electrical fields
present in the reactor. Ions accelerated to the wafer surface
may provide sufficient energy to activate chemical reactions between
the surface and gas radicals or the surface and neutral gas species.
This results in etching of the surface material. Finally, gas
molecules may capture energy from the accelerated electrons and
captured energy as a photon, or light. This last process accounts
for the glow that is characteristic of plasmas.
The TEGAL 9XX System Plasma/RlE etchers have been configured to
take advantage of the characteristics of plasmas for etching various
films. Each of the models in the 9XX family has been optimized
for specific etches of specific films. All models have the common
ability to implement multistep etch recipes using multiple process
gases. An optical monitoring system provides a means for determining
etch completion so the etch process can be terminated.
The 9XX plasma etch process chamber is a capacitively-coupled
diode operated in the RIE mode. The lower electrode is powered
by 13.56 MHz RF energy and incorporates a grounded upper electrode.
The simple fixed-gap design ensures repeatable process results
and offers simplified cleaning and preventive maintenance. The
fixed-gap electrode spacing is available in two configurations:
a 6mm or 38mm gap between upper and lower electrodes.
• The 6mm-gap reactor (9x3) confines
the plasma very closely between the electrodes, providing the
plasma potentials and ion densities necessary to break oxide bonds
at reasonable rates. This configuration is optimum for etching
silicon dioxide and other dielectric materials. Typical etchants
used are CHF3, CF4, SF6, and He.
• The 38mm-gap reactor (9X1)
causes the reactive ions to travel further, so the plasma expands,
spreading the energy throughout the full volume of the reactor
and decreasing the plasma potential. Etching in the 38mm-gap reactor,
to the 6mm-gap reactor, is accomplished more by the chemistries
involved. The 38mm-gap reactor is typically operated with SF6
or CF4/O2 etch gases. It is cleaner and minimizes plasma-induced
damage. This configuration is ideal
for isotropic, polysilicon, and silicon nitride applications,
as it is inherently selective to oxide.
Although these configurations
are optimized for specific films, most etch applications can be
successfully performed in either of the chamber configurations.
The application mix and specific production requirements dictate
which chamber is used.