XEI Scientific, Inc.
The EVACTRON® Anti-Contaminator and De-Contaminator Updated August 2007
LOW
PRESSURE NITROGEN PURGE STOPS
Ronald
A. Vane, President, XEI Scientific, June 15, 1992
Note:
EVACTRON Decontaminator system allows for both plasma cleaning and
N2 purge cleaning of SEM chambers. The following essay explains how
purge cleaning works.
Oil contamination in SEMs (Scanning Electron Microscopes) is a common problem
HOW THE
NITROGEN PURGE WORKS
A low
pressure Nitrogen purge which creates viscous flow vacuum conditions
will flush out contaminants from the SEM chamber into the roughing
pump. This technique is often used in of vacuum process equipment in
the semiconductor industry to keep the systems clean. When the SEM is
not in use, Nitrogen is leaked into the chamber during the roughing
cycle to create the purge and flushing action. The flow of Nitrogen
gives an active cleaning action that removes oil and other
contamination. When contaminants evaporate from the surfaces, the
short mean free path and flow of Nitrogen minimize the probability of
redeposition on another surface. Under high vacuum and molecular flow
conditions the oil molecules have a high probability of redepositing
on another surface, thus viscous flow is much more efficient at
removing these molecules than high vacuum pumps. The low gas flow
does not harm the rotary vane roughing pump during the purging cycles
and is beneficial to the pump by helping remove condensed water and
other gases from the oil. A manually operated nitrogen purge can be
easily be installed on any SEM, but will often be incompatible with
the automatic evacuation system of the SEM. Provisions must be made
to allow operation only during the roughing part of evacuation cycle,
and to allow the SEM to safely stay in the roughing cycle for an
extended period.
THE
SEM-CLEAN SOLUTION
A Nitrogen
purge system called the SEM-CLEAN system formerly available from XEI
Scientific worked for most models of SEMs. The SEM-CLEAN system is fully
interlocked with the evacuation control system of the SEM to prevent
accidental operation and damage to the electron gun and high vacuum
pump. The system monitors the pressure in the Specimen chamber and
allows the purge to turned on only when the SEM is in the rough
evacuation cycle. Depending on the SEM model the SEM-CLEAN system can
also turn off the diffusion pump, and make other adjustments in the
SEM evacuation cycle to safely keep the SEM in the rough evacuation
mode for an extended period.
RESULTS
AND CONCLUSIONS
The
SEM-CLEAN Nitrogen purge stops oil condensation on X-ray detector
windows almost immediately. In SEMs where regular cleaning of X-ray
window had been necessary to maintain light element sensitivity,
installation of a SEM-CLEAN system eliminated the need for periodic
cleaning immediately. Carbon buildup on specimens and dark spot
formation decays exponentially with time after installation and
disappears in three to six months. In a test of a SEM, purge equipped
for six months, a ten minute 5kV scan was done at 100 X, and then the
area examined at lower magnification. No evidence of carbon buildup
or a dark spot was seen. SEM-CLEAN systems have proved effective for
contamination control and have made cryotraps and TMPs unnecessary.
The active cleaning action of a Nitrogen purge makes it especially
suitable for applications where the samples themselves are
responsible for the contamination.
RF Plasma Cleaning Systems for Electron Microscopes
and High Vacuum Systems
Stops Artifacts and Removes Hydrocarbons and Organics.
OIL CONTAMINATION IN SEMS
The
XEI SEM-CLEAN nitrogen-purge system mentioned is no longer available.
The purge cleaning feature of the Evactron is useful whenever heavy
contamination is found in a instrument since its cleaning action is
based on a physical effect that cannot be saturated. In the Evactron
Decontaminator system purging is most useful immediately after plasma
operation to remove the ashing product gases from the chamber. Two
minutes is sufficient for this operation after Evactron plasma cleaning.
It manifests itself as dark spots and carbon build-up on the samples
in low kV scans and as oil condensation EDX detector windows impeding
light element analysis. Depending on severity, these effects are
either an irritation to the analyst or a major impediment. There are
a number of sources of contamination in SEMs including pump oils,
lubricants, seals, fingerprints, and the samples themselves. One of
the most common sources is oil backstreamed from the roughing pump. A
number of "cures" have been offered to the SEM user
including Turbomolecular pumps (TMP), cryopumps, drag pumps, traps,
better pump oils, cooler running roughing pumps, double diffusion
pumps and fractionating diffusion pumps. Most of these are either
costly to install, take too much maintenance, or are not very
effective. Now a low cost Nitrogen purge has been shown to be effective.