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Plasma
etching, also known as dry etching (in
contrast to wet etching) is the universal
tool for structure etching since about 1985. In contrast to all other
techniques around chip manufacture, which existed in some form or other before the advent of microelectronics, plasma
etching was practically unknown before 1980 and outside the
microelectronic community. |
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What is Plasma etching?
In the most simple way of looking at it, you just replace a liquid etchant by a
plasma. The basic set-up is not unlike sputtering, where you not only deposit a
layer, but etch the target at the same
time. |
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So what you have to do is to somehow
produce a plasma of the right kind between some electrode and the wafer to be
etched. If all parameters are right, your wafer might get etched the way you
want it to happen. |
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If we naively compare chemical
etching and plasma etching for the same materials to be etched - lets take
SiO2 - we note major differences: |
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| Chemical etching
of SiO2 |
Plasma etching
of SiO2 |
| Etchant: HF + H2O (for etching
SiO2. |
Gases: CF4 + H2 (or
almost any other gas containing F). |
| Species in
solution:: F–, HF–,
H+SiO42–, SiF4,
O2 - whatever chemical reactions and dissociation produces.
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Species in
plasma and on wafer: CFx+ (x
£ 3), and all kinds of unstable species not
existent in wet chemistry.
Carbon based polymers, produced in the plasma which may be deposited on parts
of the wafer. |
| Basic
processes: SiO2 dissolves |
Etching of SiO2,
formation of polymers, deposition of polymers (and other stuff) and etching of
the deposited stuff, occurs simultaneously |
| Driving
force for reactions: Only "chemistry", i.e. reaction
enthalpies or chemical potentials of the possible reactions; essentially
equilibrium thermodynamics |
Driving
force for reactions: "Chemistry", i.e. reaction enthalpies
or chemical potentials of the possible reactions, including the ones never
observed for wet chemistry, near equilibrium,
and non-equilibrium physical
processes", i.e. mechanical ablation of atoms by ions with high
energies. |
| Energy for
kinetics: Thermal energy only, i.e. in the 1 eV range |
Energy for
kinetics: Thermal energy, but also kinetic energy of ions obtained
in an electrical field. High energies (several eV to hundreds of
eV) are possible. |
| Anisotropy: None; except some possible {hkl}
dependence of the etch rate in crystals. |
Anisotropy: Two major mechanisms
1. Ions may have a preferred direction of incidence on the wafer.
2. Sidewalls may become protected through preferred deposition of e.g.
polymers
Completely isotropic etching is also possible |
| Selectivity: Often extremely good |
Selectivity: Good for the chemical component, rather
bad for the physical component of the etching mechanism. Total effect is open
to optimization. |
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If that looks complicated, if not
utterly confusing - that's because it is (and you thought just chemistry by
itself is bad enough). |
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Plasma etching still has a strong black art
component, even so a lot of sound knowledge has been accumulated during the
last 20 years. |
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It exists in countless variants, even for just
one material. |
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The many degrees of freedom (all kind of gases,
pressure and gas flux, plasma production, energy spread of the ions, ...), or
more prosaically, the many buttons that you can turn, make process development
tedious on the one hand, but allow optimization on the other hand. |
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The two perhaps most essential
parameters are: 1. the relative strength of chemical to physical
etching, and 2. the deposition of polymers or other layers on the wafer,
preferably on the sidewalls for protection against lateral etching. |
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The physical part provides the absolutely
necessary anisotropy, but lacks selectivity |
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The chemical part provides selectivity. |
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Polymer deposition, while tricky, is
often the key to optimized processes. In our example of SiO2
etching, a general finding is: |
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Si and SiO2 is etched in
this process, but with different etch rates that can be optimized |
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The (chemical) etching reaction is always
triggered by an energetic ion hitting the substrate (this provides for good
anisotropy). |
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The tendency to polymer formation scales with the
ratio of F/H in the plasma. The etching rate increases with increasing
F concentration; the polymerization rate with increasing H
concentration. |
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Best selectivity is obtained in the border region
between etching and polymer formation. This will lead to polymer formation (and
then protecting the surface) with Si, while SiO2 is
still etched. The weaker tendency to polymer formation while etching
SiO2 is due to the oxygen being liberated during
SiO2 etching which oxidizes carbon to CO2
and thus partially removes the necessary atoms for polymerization |
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Enough about plasma etching. You get
the idea. |
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A taste treat of what it really implies can be
found in an advanced module. |
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© H. Föll
