Controlled Plasma Etching

Over the past thirty years, plasma, the fourth
state of matter, has become a very useful
means of removing small quantities of
material from a variety of substrates quickly
and efficiently. Plasma processes have been
used in many highly sensitive integrated
circuit packaging and optoelectronic
applications to precisely remove specific
materials from sample surfaces. The theory
of chemical etch plasmas, some typical
advanced technology applications, and how
to control the plasma etch process for these
applications will be discussed.

THE CHEMICAL PLASMA

The room-temperature gas plasmas
employed are typically generated in a
vacuum chamber. To generate plasma, the
chamber is pumped to a pre-set base
pressure, process gas is introduced, and a
radio frequency (RF) electromagnetic field is
applied to the electrodes in the chamber
producing a glow-discharge plasma. In the
plasma, many different gaseous species are
produced. These species include ions, free
radicals, electrons, photons, neutrals, and
reaction by-products such as ozone. These
species make a highly active, lowtemperature
plasma that can etch material
selectively and quickly.
Controlling this type of plasma for effective
etching is a balancing exercise. The proper
amount of ions and free radicals (the
species that do most of the work) in and
around the area to be etched must balanced
with the RF power input into the system.
Generally, the amount of ions and free
radicals is governed by the process
pressure, thus making process pressure a
very important process parameter. RF
power input, on the other hand, must be
selected so that the highest etch rate is
produced without over etching or
inadvertently damaging other materials or
substrates. The process time parameter
must also be selected with care because it,
like power and pressure, can greatly effect
the outcome of the process cycle.
One significant advantage that chemical
plasmas have over other types of plasma is
their natural selectivity. Selectivity, in this
sense, is defined as a chemical’s propensity
to react with one substance rather than
another. In plasma, this is extremely useful
characteristic. It provides the opportunity to
tailor the plasma etching process to the
substance of interest and not cause
unwanted etching to other substances which
can be in close proximity.
Reactive Ion Etching (RIE) is a type of
chemical plasma. It is characterized not
only by the parameters and characteristics
mentioned above, but also RIE is extremely
directional and anisotropic. It has many
applications, this Application Note discusses
applications specific to semiconductor and
optoelectronic processing and packaging.

APPLICATIONS

Photoresist Removal:
Photoresist removal has two plasma
process applications. The first application is
uniform removal of small quantities of resist
over the entire surface of a wafer. This is
known as "descum." In this case, etch rate
should be moderate, and a low-reactivity
process gas, such as O2, is used. RF power
should be kept low as well. To increase the
uniformity of the descum operation,
operating pressure must remain relatively
high and usually from 600 mTorr to 1000
mTorr. At low power and high pressure, a
very isotropic and uniform distribution of ions
exists thus achieving a highly uniform,
moderately rapid etching operation.
The other plasma application for photoresist
removal is etching features from patterned
photoresist. In this case, the etching
operation must be fast and especially
anisotropic. The isotropic etch must
produce very vertical wall features with no
undercutting. As a rule, highly reactive
process gases or gas combinations such as
CF4 or a mixture of CF4 and O2 are used.
To make the plasma as anisotropic as
possible, pressure is lower than the descum
process discussed above, while RF power is
increased. The decreased pressure (usually
in the 100 to 200 mTorr range) provides the
anisotropic nature of the plasma, and the
increased power compensates for the lack
of ions to increase the etch rate. The side
effects of this operation are over etching and
non-uniformity. Process time will manage
any over etching issues, but increased
uniformity will require a rather close balance
of power and pressure. This power and
pressure balance will be material and
substrate-geometry specific and may take a
small amount of process development to
achieve satisfactory results.
Glass and Glass-like Compound Etching
In many ways, etching glass or glass-like
substances like SiO2, Si3N4, and singlecrystal
silicon is similar to photoresist
etching. The biggest difference is process
gas selection. Glass is a very non-reactive
or stable substance. Consequently, highly
reactive process gases like CF4 and SF6 are
employed. Analogous to photoresist
removal, the degree of anisotropy and
uniformity can be controlled largely by
pressure and power using the gases
mentioned. However, unlike photoresist
removal, etch rate will vary widely due to the
fact that glass is an amorphous substance
that can vary widely in composition. Care
must be taken when developing etching
recipes not to damage valuable parts by
inadvertent over etching. Applications for
this type of etch include fused-silica optical
fiber etching and etching BPSG, SiO2 and
Si3N4 in IC failure analysis operations.

Polymer Etching:
Etching polymers can be exceedingly
challenging or exceedingly simple. The
challenge springs from the fact that
polymeric substances are widely varied in
their make up. For instance, polypropylene
encompasses several hundred different
compounds of polymer, all of which conform
to the characteristics innate to
polypropylene. The slightest change in
plasticizer or UV stabilizer makes
developing a single, all-encompassing
etching recipe nearly impossible because
there is usually no common starting point.
The technique for etching polymers is the
use of a mixture of process gases. Oxygen
and tetrafluoromethane (CF4), when mixed
together for use in plasma etching, create
the oxyfluoride ion (OF-). The oxyfluoride
ion is a powerful etching agent for polymeric
substances. This ion is particularly adept at
cutting the carbon-carbon molecular bonds
in the polymer backbone and removing the
molecule quickly.
One application of polymer etching is hole
boring in polyamide when the polyamide is
sandwiched between two, conductive sheets
of metal. The holes are easily made using a
ratio 80% oxygen and 20% CF4 at low
pressure and high power. Time will depend
on the polyamide make up and the depth,
but low pressure will insure clean, straight
sidewalls in the hole. Selectivity will insure
that only the polymer inside the whole is
etched. Due to the selectivity of the etching
process, the metal around should be
unharmed by the etching operation.
Another application is the etching of
polyamide in bulk form such as off a wafer.
This, again, is similar to etching photoresist
in that the process pressure is set relatively
high for good uniformity, and the power is
increased to speed etching rate. In addition,
this operation is like etching glass because
the composition of the polyamide can be
varied, thus causing unpredictable etch
rates. Generally, a good starting point for
process recipe development is high
pressure and moderate power.
A final application in the discussion of
polymer etch is optical fiber cladding
etching. Optical fibers consist of a fused
silica core, which is approximately 100 μm
thick surrounded by polyurethane cladding
usually another 125 μm thick. The
application here is to etch the cladding
completely off a specific section of the fiber
expose the fused silica core without
damaging it. To uniformly treat all the fiber
optic strand, an especially isotropic plasma
is needed. Thus, pressure should be near
500 mTorr with high power. Time is the
most important parameter in this application,
because, even at 90% O2 and 10 % CF4, the
CF4 can damage the silica core and diminish
strand pull strength. The fiber optic stand
must be etched only long enough to remove
the cladding.

Bleedout Removal:
During epoxy dispensing operations, the
amount of dispensed epoxy is excessive or
the substrate material causes a small
quantity of the epoxy to wet out over its
surface. This problem is particularly critical
when subsequent wire bonding is required.
The epoxy residue can contaminate wire
bond pads causing poor bond strength or
wire bond "pull-up" which is the complete
failure of the wire bond. To remove the
epoxy contamination, an argon, argon and
oxygen, or argon and hydrogen plasma is
used around the 200mTorr to 250 mTorr
pressure range. Argon is an inert gas, but it
can be highly effective in removal of the
epoxy bleedout through pure bombardment
using Ar+ ions. The bombardment cleans
the surfaces of the wire bond sites by
ablating the epoxy and leaving a pristine
metal surface behind. The addition of a
chemically reactive agent such as oxygen or
hydrogen simply increases the reaction rate.
The amount of reactive gas can vary, but
usually, no more than 30% by volume is
added. There is one caveat however:
addition of oxygen can cause oxidation of
silver-filled epoxies thus turning them black.
This oxidation is nothing more than the
surface tarnishing of the silver in the epoxy,
and it does not affect the epoxy’s ability to
conduct heat or electricity.