XEI Scientific, Inc.
The EVACTRON® Anti-Contaminator and De-Contaminator Updated August 2007
How Evactron® RF Plasma Cleans
Oxidative
EVACTRON® cleaning of the SEM specimen chamber and in-situ
cleaning of the specimen gives the microscopist a new tool for
combating the persistent problem of contamination. Hydrocarbons and
other organics are oxidized by oxygen radicals (oxygen atoms) to form
H2O, CO, and CO2 gases that can be easily pumped from the SEM.
Plasma ashing and glow discharge cleaning of samples have long been a
cleaning method available for sample preparation for SEMs and TEMs,
but requires expensive auxiliary equipment. Argon and oxygen plasmas
are normally used. Argon cleans by a sputter etching mechanism.
Oxygen plasmas are more reactive, and Oxygen/Argon mixtures have
proved to be very effective for cleaning. But inside the SEM Argon
and pure Oxygen plasmas should be avoided because possible damage to
the instrument.
The EVACTRON Decontaminator (D-C) system provides a safe method for plasma cleaning inside
the SEM. Air as a diluted oxygen gas mixture is chosen as the
plasma reactant gas. Air is passed through a low-power RF
glow-discharge to create oxygen radicals inside a generator mounted
on a specimen chamber port. The generator chamber is subject to the
same vacuum as the specimen chamber, and the vacuum pressure and gas
flow are controlled by the EVACTRON SD-C controller. The radicals
are carried out of the plasma into the whole of the specimen
chamber by convection. In the chamber they react with all
exposed surfaces including the specimen if present. The plasma itself
is confined to the generator chamber, which prevents ion bombardment
damage to the instrument or specimen.
Chemistry and Thermodynamics
The
Evactron process is successful because the HC oxidation products are
volatile in vacuum. Oxygen radicals oxidize hydrocarbons and form
volatile oxides. The oxidation generally begins with hydride
extraction (hydrogen atom removal) that creates more reactive sites
on the hydrocarbon chain. These sites, when ashed by subsequent O
radicals, further breakdown the chain.
EVACTRON cleaning controls the temperature of the plasma as an
important part of the method for generation the oxygen radicals from
air. When oxygen is ionized a series of reactions lead to the
formation of oxygen radicals:
O2 + O+ --> O2+ + O
O2+
+ e- --> O + O
Compared
to the ions these radicals are long-lived species and may leave the
plasma region.
The
ionization potential of oxygen is 12.1 eV and nitrogen is 15.6 eV.
Thus oxygen ionization takes place in a lower temperature or lower
energy plasma than nitrogen. By lowering the average temperature of
the electron-energy- distribution oxygen ionization is favored. When
nitrogen ions are produced in an air plasma they react with O
radicals by the following fast reactions:
N2+
+ O --> NO+ + N
N
+ O --> NO+ + e-
Thus
two oxygen radicals are destroyed by every nitrogen ion produced.
Because nitrogen is the major constituent of air, this destruction
takes place quickly once nitrogen ionization begins. In addition, the
reaction product NO+ is a stable ion that is unable to react with the
neutral diatomic gases in air and reacts with hydrocarbons to form
nitrogen oxide polymers that are resistant to further oxidation and
removal. In EVACTRON cleaning an operating pressure and plasma
temperature are adjusted such that the oxygen radical flux to the
surfaces is maximized.
Using
EVACTRON Cleaning
EVACTRON
cleaning is a fast process, and cleaning in most cases can be
done in less than 5 minutes. After the EVACTRON process is completed
a nitrogen purge can be done for short time to flush the reactant
product gases out of the chamber before full evacuation. There are
four ways to use the EVACTRON process to clean SEMs and specimens:
·
SEM chamber cleaning: EVACTRON cleaning is used with a nitrogen
purge to clean a dirty chamber before specimens are introduced.
·
Pre-analysis cleaning. A specimen is EVACTRON cleaned inside the
SEM chamber before the electron beam is turned on to remove residual
hydrocarbons that may have been carried in on the specimen surface.
·
Mid-analysis cleaning. A specimen exposed to the electron beam
shows sign of contamination on the area of interest. The specimen is
left in place, the electron beam is turned off, and an EVACTRON
cleaning cycle performed to remove the contamination.
·
Post-analysis cleaning. After electron beam analysis the
specimen is EVACTRON cleaned to remove any contamination residues so
that the specimen can undergo subsequent analysis or use where
cleanliness is required.
The EVACTRON
system offers the microscopist a new tool for keeping his SEM
and specimens clean for better analysis, identification, and
metrology. By cleaning in-situ within the SEM it offers time saving
convenience in the pursuit of better results.
Evactron® anticontamination system is for SEM and FIBs, EDX and EDS window cleaning.
An RF Plasma Asher in Electron Microscopes that uses a
low-power, air-plasma inside the specimen chamber to create oxygen
radicals, all surfaces are reactively cleaned of hydrocarbons and
residual oils in-situ. This prevents the coating of scanned areas
with hydrocarbon polymer by the electron beam during SEM focusing or
analytical work (no black squares or
raster lines). It is designed for use microscope technicians with
only simple training. Specimen cleaning by Evactron plasma oxidation
may be done inside the specimen chamber either before or after
examination by the electron beam. In addition to plasma oxidation,
the system allows for SEM-CLEAN nitrogen purge cleaning of the SEM
chamber and vacuum system when the plasma is not on.
This cleaning
process takes place while the SEM chamber is at 0.6 Torr in
rough pumping mode and is of a short duration. (2 - 10 minutes). The plasma cleaning is activated by operator command when needed. When activated the SEM is vented first to partial vacuum and then evacuation recommenced. At the correct pressure air is bled into the plasma source and then the RF plasma is ignited to create
activated oxygen radicals for cleaning. After short time the plasma and air leak are turned off and the system is pumped down to remove the residual oxidation products. The SEM is soon ready for operation.
The
feed gas to the plasma is air that is fed into the chamber through
the exclusive
XEI designed, low-power, RF plasma generator which
maximizes oxygen radical output from air. Air is used because it is
always available, and will not create the explosive conditions in oil
sealed vacuum pumps that pure oxygen could. Argon is not used because
it may accumulate and flood the ion pumps used in many SEMs on their
electron guns.
EVACTRON Oxygen
Radical Source on SEM port
A
Nitrogen purge may be used to complete the cleaning and removes water vapor from the chamber. The Nitrogen
purge function may
be operated separately from the plasma function to suppress
backstreaming from the roughing pumps.
Oxygen
radicals are highly reactive to surface hydrocarbon scum or AMC
(Atmospheric Molecular Contamination). The volatile reaction products
of H2O, CO2, and CO
are easily pumped
away
by the vacuum system. The plasma is confined to the generator region
to minimize the bombardment of surfaces by free electrons or
energetic ions that could cause sputtering or polymerization of the
surface hydrocarbons or damage to sensitive SEM parts. The
low power
of the plasma and its confinement prevents
ion etching and damage
to detector windows and other sensitive surfaces within the SEM
chamber. The system consists of a non-sputtering, glow discharge
generator with a gas and power feed-through on a microscope port, RF
Generator and matching network, vacuum gauging and control electronics.
RF Plasma Cleaning Systems for Electron Microscopes
and High Vacuum Systems
Stops Artifacts and Removes Hydrocarbons and Organics.
SEMS, FIBS and fine film substrates:
__________
Oxidative
Cleaning in SEMS
By
Ronald Vane, President XEI Scientific
The
most thermodynamic favorable reactions are C double bond oxidation,
followed by hydride extraction, followed by C-C single bond oxidation.
C-C single bond oxidation is only slightly exothermic which accounts
for the very slow degradation of polymers compared to single chain HC
compounds. The C-C single bond is usually broken and oxidized after
an adjacent bond is oxidized to create a reactive site. In
fluorocarbons C-F bond oxidation is very endothermic and these
compounds are non-reactive.
The
effect of Evactron cleaning on other materials can be predicted by
looking at the oxidation chemistry of the target material. If a
stable oxide layer is formed as on most metals, Evactron oxidation
will stop.
__________
The
EVACTRON®
system quickly cleans the interiors of electron
microscopes including the stages, and specimens. It is an
Anti-Contamination or Decontaminator accessory for electron
microscopes and is used for hydrocarbon and organic contamination
control in vacuum systems. It is used for black square and raster
burn control, stopping oil condensation and drip from X-ray windows,
plasma cleaning of high vacuum systems. It can do remote glow
discharge cleaning, and hydrocarbon removal in vacuum chambers. The
Evactron Decontaminator is a SEM Anti-contaminator or SEM
Anticontaminator for RF Plasma cleaning and RF Plasma Ashing of
vacuum chambers, AMC (Atmospheric Molecular Contamination) removal,
or organic contamination control and removal in any high vacuum
system. It removes Organics as an decontaminator.
EVACTRON System on SEM